Plasma accelerator using hall currents



April 5, 1966 G. L. cANN PLASMA ACCELERATOR USING HALL CURRENTS 2Sheets-Sheet 1 Filed Aug. 1'7, 1962 April 5, 1966 G. L. CANN 3,243,954

PLASMA ACCELERATOR USING HALL CURRENTS Filed Aug. 17, 1962 2Sheets-Shes?I 2 RDp/J L. T4/wv,

INVENTOR United States Patent O PLASMA This invention relates to plasmaaccelerators and more particularly to electromagnetic acceleratorsutilizing Hall currents to achieve extremely high gas velocities.

Plasma generating and accelerating devices in which an ionized gasplasma is accelerated to high velocities are recognized by science topossess great potential in certain applications. A particularapplication in which such devices could be of particular importance isin the field of space propulsion. The thrust of a space propulsionsystem is dependent upon the product of the propellant mass flow rateand the exhaust velocity relative to the vehicle. The current use ofchemical rocket fuels entails a limitation on space flights because thepropellant exit velocities are relatively low and large masses ofpropellant must be carried aloft. Considerable thought has been given tothe use of an ionized gas plasma in space propulsion systems With a viewtoward increasing the exhaust velocity yof the propellant, together witha substantial reduction in propellant Weight and space requirements ascompared with chemical propellants.

The standard present art method for accelerating an ionized gas plasmato high velocities is to pass the plasma through a crossed field channelaccelerator in which electric and magnetic fields are maintained atright angles to each other and transverse to the channel into which theionized gas is longitudinally injected. It is known that an electricfield transfers energy to charged particles and that a magnetic eldexerts a force on charged particles in motion relative to the magneticfield. When a magnetic field is established at right angles to a movingstream of electrically conductive fluid, an electric field is inducedperpendicular to both the stream direction and the field lines. lf anelectric field is now applied in the same direction as the inducedelectric field, but stronger than the induced electric field, then acurrent flows in the conductive fluid in the direction of the appliedelectric field, which current interacts with the magnetic field toproduce a force which is in the direction of the moving stream. Inaccordance with commonly used vector notations, the net current densityis denoted by j and the magnetic flux density denoted by B. Hence, thecrossed-field channel accelerator is also known in the art as a jXBchannel accelerator,

However, in a B channel accelerator, the electrical current does notactually flow perpendicular to the electrodes, but rather iows at someangle due to the phenomenon known as Hall effect. When the Hallpotential is zero, the net resultant current is the combination of theordinary currents (current due to the applied electric field pluscurrent induced by movement of electrons through the magnetic field) andthe Hall cur-rents. Because of the angularity of the net resultantcurrent flow between the electrodes, a force is applied to the gasstream deflecting the flow in the direction of the applied electricfield, the angle of deflection changing with variations in gas pressureand in the strength of the applied magnetic field. Thus, acceleration ofa plasma jet in accordance with present art practices results indeflection as well as spreading of the jet, these characteristics beingundesirable for use of the jet as a space propulsor because maximumthrust is obtainable only from a sharply focused jet containing noangular velocity components.

Patented Apr. 5, 1966 Also, in the present art j XB channelaccelerators, energ is initially transferred from the electric field tothe electrons and must then fbe transferred by collisions to the heavierparticles. Due to the slow rate of energy transfer between electrons andheavy particles by elastic collisions, the electrons usually are heateduntil they can collide inelastically and ionize the atoms, therebytransferring a large fraction of the input electrical energy into a formof potential energy that is not easily recoverable. There is also a veryhigh heat iiux into the electrodes, causing them to erode at arelatively high rate.

The present invention is directed toward obviating the aforementioneddisadvantages of present art channel accelerators by accelerating anionized gas plasma without causing deceleration `or spreading of the jetwhile maintaining the electrodes at a relatively low temperature, thejoule heating of the gas being converted into axial jet energy in thesame region where the acceleration is 0ccurring. The spiraling forces ofthe ordinary currents are minimized and the Hall currents utilized toprovide an axial accelerating force.

Accordingly, it is an object of the present invention to provideimproved plasma accelerators.

lt is also an object of the present invention to accelerate an ionizedgas plasma without any significant spreading or deflection of the jet.

It is another object of the present invention to provide anelectromagnetic plasma accelerator in which the electrodes aremaintained at a relatively low temperature to minimize electrode erosionand sputtering.

lt is a further object of the present invention to provide a steadystate plasma accelerator in which Hall currents are used to provide anadditional measure of acceleration.

It is yet another object of the present invention to provide an improvedplasma accelerator in which the joule heating of the gas plasma isconverted into axial jet energy.

lt is a still further object of the present invention to provide animproved plasma accelerator in which tangential velocities areeffectively cancelled by clockwise and counterciockwise accelerationcomponents.

lt is also an object of the present invention to providel methods andmeans for focusing a plasma jet and eliminating the angular momentum ofthe jet while accelerating it to high velocities.

The objects of the present invention are accomplished by partiallyionizing a gas stream, accelerating the jet to supersonic velocitiesthrough a nozzle, and accelerating the jet through axisymmetric fringemagnetic fields and a secondary electrical discharge extending axiallydownstream throughout the length of the fringe magnetic fields.Tangential Hall currents are generated by the current density linescrossing the magnetic field lines, the Hall currents then interactingwith the radial components of the magnetic .eld to impart axialacceleration to the gas.

field. in the accelerating discharge, the electrons are forced to spiraldue to the radial component of the applied. fringe magnetic fields, therotational force appliedV to the electrons upon entering the fringe eldbeing oppositelydirected from the rotational force'applied on leavingthe fringe field. ience, the rotational forces cancel each other,thereby leaving only the axial Hall force for a0- celeration of the jet.By establishing a series of separate axisymmetric magnetic fringe fieldsalong the discharge path, even greater exhaust velocities can beachieved since the jet is acceleratedby the axial Hall forces createdwithin each magnetic field. Although the electrons spiral within thefringe magnetic field the ions, being of much greater mass, areunaffected by the rotational forces and The Hall electric field iseffectively eliminated due to the impossibility of maintaining atangential electric so carry the current and pick up momentum and energydirectly from the electric elds. The ions simultaneously collide withthe atoms and hence accelerate the plasma as a whole. Y i The novelfeatures which are believed to be characteristic of the Vpresentinvention, together with further objects and advantages thereof, will bebetter understood from the following description considered inconnection with the accompanying drawing in which the invention isillustrated by Way of example. It is to be expressly understood,however, that this description and the drawing are for the purposes ofillustration and description only, and that the true spirit and scope ofthe invention is defined by the accompanying claims.

In the drawing:

FIGURE 1 is an elevation view, in section, of a first embodiment of aplasma accelerator;

y FIGURE 2 is a schematic diagram defining thevarious angles used in themathematical analysis presented in the specification; Y

FIGURE 3 is an elevation view, in section,'of a second embodiment of aplasma accelerator; and,

FIGURE 4 is an elevation view, in section, of a third Y Aembodiment of aplasma accelerator. Y

In general, electric discharges in gases tend to operatemost'satisfactorily in an axial symmetric configuration.

'Hence, the present invention concepts are applied to a 'Y in a tubularcasing 1i) of an insulating material. Coaxial- 1y disposed Within thetubular casing 10, near one end thereof, are a cathode 11, anintermediate shaping electrode`12, and an anode 13. The intermediateelectrode 12Y and the anode 13 are maintained in coaxial alignment andelectrically insulated from each other by a tube 16 of an electricalinsulating material.V The cathode 11 is mounted within'a cylinder `17 ofinsulating material.

The cathodell is cylindrical in shape and defines a pointed end portion11a. The cathode is constructedof a suitable metal, tungsten beingpresently preferred. The cathode 11 is coaxially encased within the'insulating cylinder 17 with its pointed end portionfla extending pastthe end of the insulating cylinder. A multi-branchgas inlet Vpassageway18 extends coaxially into the cathode 11V from an inlet port 19, withthe'various branches of the passageway extending radially outward andinto the in-v sulating cylinder. 17 and then longitudinallywithin thecyllnder toits innermostYYend. One end of a gas feedplpe Ztl is coupledto the inlet port 19, the other end of Y Y the pipe. being coupled toa'suitable source of gas pressure, not'shown. i a .Y

The intermediate electrode 12 defines an angular end of the cathode 11.The frusto-conical section 25b denes a super-sonic expansion nozzle 27.The end section 25a of the conduit means 25 is positioned adjacent theflange portion 13a defining the ring anode and separatedY therefrom byan insulating ring 28.

The open volume defined Ibetween the pointed endportion of theY cathode11 and the end section of the conduit means 25 forms a gas chamber.Plasma is produced within the chamber by pumping gas under pressurethrough the inlet passagewayV Iinto the chamber' and through an arcmaintained between the cathode 11 and the anode 13, the plasma -thenpassing through the'sonic z Yorifice 26 and expanded vin the supersonicnozzle 27.

The design and construction'of such arc gap devices are. well known inthe art and hence will not be discussed in detail.

The end of the tubular casing Vliti adjacent the outlet. of the nozzle27 defines a radially inwardly extending flange 10a, the flangeterminating flush with the mouthy 'of the nozzle. i Disposed adjacentthe flange lllais va'ring i.

cathode 30 fabricated of copperor other suitable electrically conductivematerial. The cathode V30 has atubu-Y lar extension 30a, by means ofwhich the ring cathode lar casing 10. Wound about the tubular casing Iiiisfa magnetic field coil 35, the field coil comprising multiple layersYof coil windings wound circumferentially about the tubular casing llland extending throughout the length lof the arc -gap devicewithin-thecasing and past theL a orifice of the expansion nozzle; The `held coil3S1sen cased within a ferromagnetic shield 36.Which serves to establishthe axisymmetric flux path shownby the dotted rotating magnetic field isestablished along the lengthofV the accelerator, the lield strengthbeing merely adequate; Y

to prevent a current filament frornforming along the u axis. Thetransverse rotatinglmagnetic field is provided portion 12a projectingradially inward past theV pointed Y end portion 11a of the cathode 11for the purpose of directinggas emerging frornthe multi-branch passage-VY Y Way 18 radially inward past the pointed end portion 11a and throughthe aperture defined by the angular end portion'12a.V TheinsulatingVtube 16 defines a radiallyfinwardrly directed'ange 16a. The generallytubular anode 13 defines a radially inwardly directed ange portion 13a,

' the ange portion 13a extending paYst the flange 16a to define a ringanode. TheV intermediate electrode 12 and the anode 13 are constructedAof a suitable electrically Y conductive material, copper being presentlypreferred.

- Also disposed within the tubular casing 10 is a generally tubularconduit means 25 having a transversely extending end section 25a and adivergent frusto-conical Vsection 25b. The end section 25a defines asonic orifice 26 in coaxial alignment with the pointed end portion 11aby a plurality of coils 38 circumferentially disposed about theexpansion nozzle. e

v In the operation of the device shown 'in FIGURE l, an arc jet isutilized to heat andV partially Yionize a gas flow which is then passedthrough the sonic orifice 26 kand expanded in theYsupers'onic nozzle 27aA Vsecond electric discharge Vis maintained along the length oftheVnozzle by striking a discharge from the Yring cathode 33V at the nozzlevoutlet to the anode 13` ofthe first arc. An axisymmetric magnetic fieldis applied, throughout the Vvolume of the nozzle Ysuch that thefieldstrength drops to a very small value 'at the nozzle outlet. A tangentialY Hall current will be induced ,that willY interact with the Y appliedmagnetic field to produce axial and radial forces Y on the gas.V Therewill also be'apositive axial accelerating force on theV gas due to theinteraction of the radial.Y component of the current with theVtangential magneticiV field induced by the axial current. Thereare'four' ac- YY celerating mechanisms acting upon the plasma'passingthrough the accelerator structureofFIGURElY;First,

there is supersonic joule heating with expansion. Sec-y r ematicalanalysis of this apparatus, it will beV assumedV v that .the tangentialmagnetic field is negligibly small. As

long as the applied field is -ov'er'several hundred gauss,`

this is a reasonableassumption when the axial curren is ofthe order ofaYfew hundred aniperes.V Y 'Y r is securely maintained in coaxialalignment on the .tubu- Y Third, an axial volume Y The equationsdescribing the current density are as follows:

and where the subscripts e, I, a, respectively refer to electrons, ions,and atoms.

lt is possible to solve exactly for the three components of the forceper unit volume of gas by defining the angles as shown in FIGURE 2 ofthe drawing, wher-ein gb represents the angle of the magnetic field withrespect to the axis. It is seen that as lonfy as a l, a good solutionwill be obtained. The ideal solution is obtained when aeszll. Theresults are shown below:

ln these equations, u, v, and w are the components of the mass velocityvector in the representative r, and Z directions. The expression FZ canbe greatly simplified if it is assumed that the pressure and magneticfield are chosen so that (win-Q2 2 i/2111 a lltwerewrfr-i-LQTECUITI Smd) l tan qb 1 and wererl l (l0) then raefao on when EZ is .the rathercomplicated back term 6 shown in the Equation 6. Subject to theconditions indicated in Equations l0 and 9, a number of importantconclusions can be drawn from Equation 5a.

(l) The axial acceleration is always in the direction of the appliedelectric field and is proportional .to the potential drop of thedischarge through the nozzle.

(2) The axial acceleration is independent of the direction of theapplied magnetic field subject to the restriction of Equation 9.

(3) As long as w1r l, the magnitude of the axial acceleration isindependent of the magnitude of the applied magnetic field. However, themagnetic lield must be strong enough so that the conditions specified inEquations 10 and 9 are maintained. rThus, magnetic field interactions,rather than gas dynamic forces, will develop the main axial acceleratingforce for the gas plasma. Therefore, it can be seen that the desiredpredominance of magnetic field interactions can be achieved bymaintaining wfr greater than unity for the electrons in the plasma andless than unity for the ions in the plasma.

ln addition to the axial acceleration, there are radial and tangentialforces on the gas as indicated by Equations 8 and 7. When cot 0, theradial force on the charged particles is directed inward when the axialforce is accelerating the gas. ln practice, this signifies that the beamof charged particles would contract when emerging from an axisymmetricmagnetic field and would expand when entering such a field. Thetangential component of the body force will induce a considerable amountof rotation in the gas. Jthen the gas is continually expanding, some ofthis rotational energy will be recovered due to the necessity ofconserving angular momentum.

Thus, in the accelerator embodiment shown in FIF- URE l of the drawing,the tangential component of the body force, when the gas is expandedthrough the nozzle, is utilized to increase the total axial force orthrust developed. There is, however, a much better method of increasingthe total axial thrust. As shown by Equation 7, it is possible toreverse the angular acceleration by reversing the angle y). As long asthen it should be possible -to accelerate the gas axially and impartvery little angular velocity to it by utilizing an axisymmetric magneticfield that fringes strongly at both the inlet and the outlet, i.e., anaxisymmetric magnetic field in which the total radial components of themagnetic lines of force greatly predominate over the axial components. Aplasma accelerator utilizing this principle is illustrated in FIGURE 3.The accelerator embodiment of FlGURE 3 utilizes the basic arc gap deviceand expansion nozzle of FlGURE l, together with the axisymmetric fieldcreated by the field coil 35 and the ferromagnetic shield 3 To thisbasic structure has been added a fringe field coil 4t) and a pluralityof coils 4i and 42. The coils 4l are circumferentially spaced around theconduit means 25 between the end section 25a and the fringe field coilAlti. The coils 42 are circumferentially spaced around the conduit means25 on the other side of the fringe field coil at?. The flux lines of thefringe field produced by the field coil 46 are shown in FIGURE 3 as aseries of dot-dash lines. The magnetic field created by the coils 41 and4t2 are elds which rotate around the circumference ofthe conduit means25.

As the plasma enters the fringe magnetic field, there are components ofacceleration in the following directions:

(l) A positive axial acceleration.

(2) A radial outward acceleration.

(3) A counterclockwise angular acceleration. As the plasma leaves thefringe magnetic field, it again receives a positive axial acceleration.However, now the radial force is directed inwardly and the tangentialacceleration is clockwise. By proper adjustment of the fringe androtating magnetic iields, it is possible to focus the plasma into awell-defined jet at the mouth of the nozzle with negligible angularvelocity components in the jet. Thus, the fringe magnetic field createdby the field coil 40 provides an axial velocity increment to the plasmajet. An extension ofV this principle would be to utilize a plurality offringe field coils to obtain an axial velocity increment from each'ofthe fields. FIGURE 4 of the Vdrawings shows such an acceleratorembodiment.

` The accelerator of FIGURE 4 is the basic embodiment of FIGURE 1 towhich it has been added a plurality of magnetic r'ield'co'ils 50 encasedwithin a metal'tube 52, the tube 52 forming an extension of the channelthrough which 'the plasma is accelerated and replacing the ring cathodeof FIGURE 1. The coils 50 are relatively closely spaced and arecyclically displaced from the centralV axis of the device in 90 steps toprevent a region 'of very low radial magnetic iield from occurring alongthe center line to thereby prevent formation of a current filament alongthe central axis. Thus, the coils 50 provide the dual function of aplurality of fringing magnetic fields and rotating magnetic fields. Themetal tube 52 defines a radially inwardly extending flange 52a toprovide an outlet nozzle for the device and airing cathode forestablishing the aforementioned second electric discharge to the anode13.

The hereinabove-described accelerator embodimentsV possess anumber ofpractical advantages over the channel jXB accelerator and'other steadystate accelerating devices: Y

(l) The present invention structures obviate the prob- Vlern ofVintroducing a plasma into amagnetic iield of an accelerator and thenremoving it from the fieldV without causing deceleration or spreading ofthe jet.

(2) The joule heating of the gas is converted into axial jet energy inthe expansion nozzle/in the same region where the acceleration isoccurring. Y

(3) The tangential velocities are cancelled by both Y clockwise andcounterclockwise acceleration, hence, the

back is minimized.V In any case, independent of the magnitude of theback E.M.F., the axial electric eld canradjust itself to maintain theaxial discharge at all times. Hence, Yit should be possible to give anarbitrarily large velocity increment to the gas plasma by using multiplestages of acceleration. Y

' (4) The current from the accelerating discharge enters i the anodewhere the gas is comparatively cool and the pressure is high. ,For thisreason, the electron energy at theY anodeI surface is only a fraction ofan electron volt.

Moreover, the anode point of attachment is rotating Y ,rapidly due tothe axial magnetic iield in this region of cthearc; All of these factorstendto prevent local heating of Vthe anode surface, and hence anodeerosion and sputtering is minimized. Y n

(5) The plasma jet is axially focused and angular mo-V mentum componentsare minimized, thereby rendering these accelerators especially suitedfor space propulsion.

The gases utilized in the present invention acceleratorsVVV shouldpossess several distinct characteristics. The gases should have arelatively low ionization potential (less than about 16 volts) and amolecular weight greaterY than radians per collision.

e about 40. The gases should Vbe non-corrosive, non-oxidizing, i.e.,gases Vwhich will not attack metallic surfaces Whether the gases are inionror atomic form. Examples of gases possessing these desirableVcharacteristics are argon, nitrogen, cesium, and lithium.Alternatively,aV

between the anode 13 and the iirst cathode 1`1is on theV order from 40to 150 volts, the power in the pre-ionizing arc being within therange'from about 2 to 50 kw. The primary purpose of the first are is toionize rather than to heat the gas. As stated hereinabove, there is nopotential applied to the intermediate shaping electrode 12,.` itsfunction being merely to channel the gas. The second cathode (ringcathode 3i) in FIGURES 1V and 3 and cathode 52 in FIGURE 4) ismaintained Within the range of from about 200 to 1,000 volts below theanode, and therefore below the rst cathode. The second cathode is at theoutlet end in all of the embodiments andY is insulated from theVremainder of the charge. The discharge set up by the potential betweenthe second cathode and the anode extends along the conducting channel,the current of this discharge being carried by the ions. VThe ions arebeing axially accelerated through the electrostatic potential and arebeing tangentially accelerated in a spiralling pattern by the radialcomponent of the magnetic fields, the direction of the spiraldependingupon the direction of the radial component of the magneticVfields which varies from coil to coil. Therefore, the tangentialvelocity component isy cancelled out.Y An exhaust1 velocity of from20,000 to 50,000 meters per'second, or higher, is therefore obtained.

The gas is pumped in through the inlet port 19 under Y a pressure on theorder from 2 to 3 atmospheresi The' gas is ionized Within thechamber'and formed into a plasma and injected into the sonic nozzle Withsupersonic expansion. VThe plasma flows along the channel and is actedupon by the second discharge and the fringe magnetic fields, Thedischarge current is carried byY theionsY Y while the electrons arespiralling as theyV enter the channel.

the direction of the radial component of the alternating magneticfields. VThus; althoughV the ions areV being axially accelerated throughan electrostatic potential, they are also being tangentially acceleratedin a spiral pattern by the radial component of the magnetic fields. VTheradial component of the magnetic field is adjusted to can-V cel thetangential velocity component.. Thus, there has been described a novelplasma accelerator concept wherein Hall currents are utilized toVVsignificantlyV increase the axial acceleration of a gas"-V plasma. kByinjecting a plasma'axi'allyY into afchannel along which is maintained anaxial electric iield-and a.

` strongly fringing axisymmetric magnetic field, the plasma through thechannel.

is subjected to axial and rotational forces as it passies Interactionbetween the applied (axial) electric field and the fringe (radial)magneticV field produce tangential Hall currents. These tangentialV Hallcurrents provide an axial force on the gasrplasma. teraction betwen thetangential Hall currents and Vthe radial components of the fringemagnetic fields provide rotational forces on the gasV plasma." However,since the Y'radial components of the magnetic lines of forceV atA oneend of the fringe field are oppositely directed from those at the otherend of the fringe iield,.the rotational forces Vapplied to the gasplasma as it enters the fringe field are oppositely directeditrom therotational forces applied to the plasma as it leaves the fringe field.VHence, the rostood thatthe present disclosure has been made only by wayof example and Vthat numerous changes in the de- Y tails of constructionand the combination and arrangementof parts may be resorted to Withoutdeparting fromY The direction of electronY spiralling is reversed as Ythe electrons traverse the channel, in accordance with theV spirit andscope of the invention as hereinafter claimed.

What is claimed is:

1. A plasma accelerator comprising, in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle thereinextending longitudinally between a sonic orice inlet and a nozzleoutlet, the longitudinal axis of said nozzle defining a reference axisfor the accelerator;

(b) means for at least partially ionizing a gas stream and injecting theresulting gas plasma axially into the sonic orice inlet of said nozzle;

(c) means for establishing an axisymmetric electric discharge extendingaxially through said nozzle to thereby establish a plasma jet definingan axial current flow through said nozzle; and

(d) means for establishing a magnetic field within at least apredetermined longitudinal portion of said nozzle, saidV magnetic fieldbing axisymmetric with respect to said reference axis `and stronglyfringing intermediate said sonic orifice inlet and said nozzle outlet sothat the total radial components of the magnetic lines of force of saidmagnetic field greatly predominate over the axial components of themagnetic lines of force of said magnetic field and cross lthe currentdensity lines of the axial current fiow of said plasma jet through saidaccelerator channel.

2, A plasma accelerator comprising, in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle thereinextending longitudinally between a sonic orifice inlet and a nozzleoutlet, the longitudinal axis or said nozzle defining a reference axisfor the accelerator;

(b) an arc gap device disposed within said casing adjacent said sonicorifice inlet for at least partially ionizing a gas stream and iniectingthe resulting gas plasma axially into said sonic orifice inlet, said arcgap device including a first cathode electrode and an anode electrodebetween which a first electric discharge is maintained to partiallyionize said gas stream;

(c) meansror maintaining a second electric discharge axially throughsaid nozzle between said nozzle outlet and the anode of said arc gapdevice to thereby establish a plasma jet defining an axial current iiowthrough said nozzle; and

(di) means for establishing a magnetic field within at least apredetermined longitudinal portion of said nozzle, said magnetic fieldbeing axisymmetric with respect to said reference axis and stronglyfringing intermediate said sonic orifice inlet and said nozzle outlet sothat the total radial components of the magnetic lines of force oi' saidmagnetic eld greatly predominate over the axial components of themagnetic lines of force of said magnetic field and cross the currentdensity lines of the axial current flow of said plasma jet through saidnozzle, the field strength of said magnetic fielr1 being suiiicientlystrong with respect to the ambient pressure in said nozzle so that themagnetic field interactions predominate over the gas dynamic forces toaxially accelerate said plasma jet through said nozzle.

3. A plasma accelerator comprising in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle thereinextending longitudinally between a sonic orifice inlet and a nozzleoutlet, the longitudinal axis of said nozzle defining a reference axisfor the accelerator;

(b) an arcv gap device disposed within said casing adjacent said sonicorifice inlet for at least partially ionizing a gas stream and injectingthe resulting gas plasma axially into said sonic orifice inlet, said arcgap device including a first cathode electrode and an anode electrodebetween which a first electric disl@ charge is maintained to partiallyionize said gas stream;

(c) means-for maintaining an axial second electric discharge throughsaid nozzle between said nozzle outlet and the anode of said arc gapdevice to thereby establish a plasma jet defining an axial current flowthrough said nozzle, said means including a second cathode electrodepositioned at said nozzle outlet and a source of electric potentialconnected between said second cathode electrode and said anode; and

(d) means for establishing a magnetic field within at least apredetermined longitudinal portion of said nozzle, said magnetic fieldbeing axisymmetric with respect to said reference axis and stronglyfringing intermediate said sonic orifice inlet and said nozzle outlet sothat the total radial components of the magnetic `lines of force of4said magnetic field greatly predominate over the axial components ofthe magnetic lines of force of said'rnagnetic field and cross thecurrent density lines of the axial current flow of said plasma jetthrough said nozzle, the field strength of said magnetic held beingsufficiently strong with respect to the ambient pressure in said nozzleso that the magnetic field interactions predominate over the gas dynamicforces to axially accelerate said plasma jet through said nozzle.

4.. A plasma accelerator comprising, in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle thereinextending longitudinally between a sonic orifice inlet and a nozzleoutlet, the longitudinal axis of said nozzle defining a reference axis`for the accelerator;

(b) an arc gap device disposed within said casing adjacent said sonicorifice inlet for at least partially ionizing a gas stream and injectingthe resulting gas plasma axially into said sonic orifice inlet, said arcgap device including a first cathode electrode and an anode electrodebetween which is established a first pred-etermined potentialdifiere-nce sufficient to maintain an electric discharge to partiallyionize said gas stream, said arc gap device further including means forestablishing a first magnetic field having its lines of force extendinggenerally axially through said arc gap device and through said sonicorifice inlet;

(c) means for maintaining a secondelectric discharge between said nozzleoutlet and the ano-deof said are gap device to thereby establish aplasma jet defining an axial current flow through said nozzle, saidmeans including a second cathode electrode positioned at said nozzleoutlet and a second predeterminedpotential difference maintained betweensaid second cathode electrode and the anode of said arc gap device, saidsecond predetermined potential difference being substantially greaterthan said first predetermined potential difference; and

(d) means for establishing a second magnetic field within at least apredetermined longitudinal portion of said expansion nozzle, said secondmagnetic field being axisymmetric with respect to said reference axisand strongly fringing intermediate said sonic orifice inlet and saidnozzle outlet so that the total radial components of the magnetic linesof force of said second magnetic field within said nozzle greatlypredominate over the axial components of the magnetic lines of force ofsaid second magnetic field and cross the current density lines of theaxial current flow of said plasma jet through said nozzle, the fieldstrength of said second magnetic field being sufficiently strong withrespect to the ambient pressure in said nozzleso that the magnetic fieldinteractions predominate over the gas dynamic forces to axiallyaccelerate said plasma jet through said nozzle.

rSfA plasma accelerator comprising in combination: A(a) a casing havingan axisymmetric supersonic exjacent said sonic orifice inlet for atleast partially Y ionizing Va gas stream and injecting the resulting gasplasma axially into said sonicY orifice inlet, said arc gap deviceincluding a first cathode electrode and an anode electrode between whichis established a first predetermined potential difference sufficient tomaintain an electric discharge to partially ionize said g-as stream,said arc gap device further including means for establishing a firstmagnetic field having itslines of force extending generally axiallythrough said arc gap device Vand through said sonic orifice inlet; (c)means for maintaining a second electric discharge between said nozzleoutlet and the anode of said arc gap device to thereby establish aplasma jet de- Yining an axial current fiow through said nozzle, saidmeans including a second cathode electrode positioned at said nozzleoutlet and a second predetermined potential difierence maintainedbetween said K second cathode electrode and the anode of said arc Vgapdevice, said second predetermined potential difference beingsubstantially greater than said first predetermined potentialditference; and (d) means for establishing ka second magnetic fieldYwithin a first predetermined longitudinal portion of l said nozzle,said second magnetic field rotating about Vsaid reference axissubstantially transversely thereto;

and i Y' (e) means for establishing a-third magnetic field within asecond predetermined longitudinal portion of said nozzle, said thirdmagnetic field being axisymmetric With respectlto said reference axisand strong- 'ly fringing so that the total radial` components of themagnetic lines of force of said third magnetic field greatly predominateover the axial components of the magnetic lines of force of said thirdmagnetic field and cross the current density lines of the axial Ycurrent flow of said plasma jet through-said nozzle, the field strengthof said thirdrmagnetic field being sufficiently strong with respect tothe ambient vpressure in said nozzle soY that the magnetic fieldinteractions within said nozzle predominate-over the gas Y nozzledefining a reference axis -fcr the accelerator; (b) an arc gap devicedisposed within said casing between said first end andtsaid sonicorifice inlet for v at least partially ionizing a gas stream andinjecting the resulting gas plasma axially into said sonic orificeinlet, said arc gap device including a first cathode electrode and ananode electrode between which is maintained a first predeterminedpotential difference sufficient to maintain an electric dischargetherebetween to partially ionize said gas stream, said arc gap devicefurther including means for establishing a first'magnetic field havingits lines of force extending generally axially through said arc-gapdevice and throu-gh said sonic orifice inlet; Y

(c) means for maintaining an axial secondelectrick discharge lbetweensaid second end of said casing fand the anode of said arc gap device toYthereby establish a plasma jet defining an axial current flow Vthroughsaid casing, said means including a second cathode electrode positionedat said second,` end Vof said casing and a second predeterminedpotential difference maintained between said second cathode electrodeandtheA anode of'said arc gap device, said second predetermineddifference being substantially greater than said firstpredetermined'potential difference, said second cathode electrodedefining a circular opening therethrough Vcoaxial with saidpredetermined axis; and t (d) means for establishing a plurality offringe magnetic fields extending through predetermined adjacentlongitudinal portions of said casing inter-mediate said nozzle outletand said second cathode electrode, each of said fringe magnetic fieldsbeing axisymmetric withV respect to an axis parallel to said referenceaxis and angularly displaced from the refer- Vence axes of theimmediately adjacent Vmagnetic fields, each of said tfringeA magneticfields being strongly fringing so that the total radial components ofthe magnetic lines of yforce of each of said fringe magnetic fieldsgreatly predominate over the .axial components of the magnetic lines offorce of Veach of said fringe magnetic fields and cross the current Y Yand injecting the resulting gas plasma axially intoV the inlet of saidchannel;

(c) means yfor establishing lan axisymmetric electric dischargeextending axially through said channel between the inlet and outlet ofsaid -channel to-thereby Vestablish a'plasma jet defining an axialcurrent flow through said channel; `and Y 1 I Y (d) means 'forestablishingamagnetic field within at least a predetermined longitudinalportion bf said channel, said magnetic field being V*axisymmetric withyrespect `to said reference axis and strongly ringing intermediate saidinlet and said` outlet so that the total radial Ycomponents of the'magnetic lines'of force of saidmagnetic'eld greatly predominate overthe axial components of the magnetic lines of Yforce of said magneticfieldV and cross the currentV density lines of the axial current flow ofsaid plasma jet through said accelerator channel, the field strength ofsaid magnetic eld being sufiiciently strong with respect to theambientpressure in said Vaccelerator channel so that the magnetic-fieldinteractions predominate over the lgas dynamic forces to axially acvcelerate said plasma channel. Y Y t 3. A plasma accelerator comprisingin combination: (a) a casing having a tubular channel therein extendjetthrough said accelerator ing longitudinally between an inlet and anoutlet, theY (c) means for establishing-apotential differencev betweenthe inlet and outlet of said channel sufficient to'maintain 'an axialelectric discharge therebetween to thereby establish a plasma jetdefiningV an axial -current fiowV through said channel;VV i Y l' (d)means forie'stablishing a first magnetic eld within a firstpredetermined longitudinal portion of saidV channel, said first magneticfield rotating about said reference axis substantially transverselythereto; and

(e) means for establishing a second magnetic field within a secondpredetermined longitudinal portion of said channel, said second magneticfield being axisymmetric with respect to said reference axis andstrongly fringing so'that the total radial components of the magneticlines offforce of said second magnetic field greatly-predominate overthe axial components of the magnetic lines of force of said secondmagnetic eld andv cross the current density lines of the axiall currentfiow of said plasma jet through said channel, the field strength of saidsecond magnetic field being sufliciently strong with respect to ambientpressure in said channel so that the magnetic field interactionspredominate over the gas dynamic forces to axially accelerate saidplasma jet through said channel.

9. A plasma accelerator comprising in combination:

(a) a casing having a tubular channel therein extending longitudinallybetween an inlet and an outlet, the longitudinal axis of said channeldefining a reference axis for the accelerator;

(b) means for at least partially ionizing a gas stream and injecting theresulting gas plasma axially into the inlet of said channel;

(c) means for establishing a potential difference between the inlet andoutlet of said channel sufficient to maintain an axial electricdischarge therebetween to thereby establish a plasma jet defining anaxial current flow through said channel; and

(d) means for establishing a plurality of magnetic fields extendingthrough predetermined adjacent longitudinal portions of said channel,each of said magnetic fields being axisymmetric with respect to an axisparallel to said reference axis and angularly displaced from thereference axes of the immediately adjacent magnetic fields, each of saidmagnetic fields being strongly fringing so that the total radialcomponents of the magnetic lines of force of each of said magneticfields greatly predominate over the axial components of the magneticlines of force of each of said magnetic fields and cross the currentdensity lines of axial current fiow of said plasma jet through saidchannel, the field strengths of said magnetic fields being sufficientlyIstrong with respect to the ambient pressure in said channel so that themagnetic field interactions predominate over the gas dynamic forces toaxially accelerate said plasma jet through said channel.

if?. A plasma accelerator comprising in combination:

(a) a casing having a tubular channel therein extending longitudinallybetween an inlet and an outlet, the longitudinal axis of said channeldefining a reference axis for the accelerator;

(b) an arc gap device disposed within said casing adjacent said channelinlet for at least partially ionizing a gas stream and injecting theresulting gas plasma axially into the inlet of said channel, said arcgap device including a first cathode electrode and an anode electrodebetween which a first electric discharge is maintained to partiallyionize said gas stream;

(c) means for maintaining an axial second electric discharge between theoutlet of said channel and the anode of said arc gap device to therebyestablish a plasma jet defining an axial current fiow through saidaccelerator channel; and

(d) means for establishing a magnetic field within at least apredetermined longitudinal portion of said channel, said magnetic fieldbeing axisymmetric with respect to said reference axis and stronglyfringing intermediate said inlet and said outlet so that the totalradial components of the magnetic lines of force of said magnetic fieldgreatly predominate over the axial components of the-magnetic'lines offorce of said magnetic field and cross the current density linesV ofthel axial current flow of said plasma jet through said acceleratorchannel, the field strength of said magnetic eld being sufficientlystrong with respect to the ambient pressure in said accelerator channelso that the magnetic field interactions predominate over the gas dynamicforces to axially accelerate-said plasma jet through said acceleratorchannel.

11. The plasma accelerator device as defined in claim 16 wherein saidmeans for maintaining said second electric discharge between the inletand outlet of said channel includesv a second cathode electrodepositioned at said channel outlet and a source ofi electrical potentialconnected between said second cathode electrode and the anode electrodeof said arc gap device.

12. A plasma accelerator comprising in combination:

(a) means for at least partially ionizing a gas stream and directing theresulting gas plasma -along a reference axis past a first predeterminedpoint;

(b) means for establishing a potential difference along said referenceaxis between said first predetermined point and a second predeterminedpoint sufiicient to maintain an electric discharge therebetween tothereby establish a plasma jet defining an axial current flow along saidreference axis between said first and second predetermined points, saidsecond predetermined point being spaced away from said rst predeterminedpoint in the direction of gas fiow; and

(c) means for establishing a magnetic field axisymmetric with respect tosaid reference axis and strongly fringing intermediate said first andsecond predetermined points so that the total radial components of themagnetic lines of force of said magnetic field greatly predominate overthe 4axial components of the magnetic lines of force of said magneticfield and cross the current density lines of the axial current fiow ofsaid plasma jet between said first and second predetermined points, thefield strength of said magnetic field being sufficiently strong withrespect to the ambient pressure along said reference axis between saidfirst and second predetermined points so that the magnetic fieldinteractions predominate over the gas dynamic forces to axiallyaccelerate said plasma jet from said first predetermined point to saidsecond predetermined point.

13. A plasma accelerator comprising in combination:

(a) a casing having a tubular channel therein extending longitudinallybetween an inlet and an outlet, the longitudinal axis of said channeldefining a reference axis for the accelerator;

(b) an arc gap device disposed within said casing adjacent said channelinlet for at least partially ionizing a gas stream and injecting theresulting gas plasma axially into the inletof said channel, said arc gapdevice including an anode a predetermined distance from said inlet and afirst cathode electrode disposed adjacent said inlet, said first cathodeelectrode defining a circular opening therethrough coaxial with saidreference axis, said arc gap device further including means forestablishing a first magnetic field having its lines of force extendinggenerally axially through said arc gap device and through the opening insaid ring cathode;

(c) means for establishing an axisymmetric electric field extendingaxially between the outlet of said channel and the anode of said are gapdevice to thereby establish a plasma jet defining an axial current fiowthrough said channel, said means including a second cathode electrodepositioned at said channel outlet and a source of electric potentialconnected between said second cathode electrode and said anode, saidsecond cathode electrode defining a cir- 15 cular opening therethroughcoaxial with said predetermined axis; and (d) means forrestablishing asecond magnetic field within at least a predetermined longitudinalporbient pressure in said channel so that the magneticY fieldinteractions predominate over the gas dynamic forces to axiallyaccelerate saidplasma jet through said channel. Y f

tion of said channel, said second magnetic eld being axisymmetric withrespect to said reference axis and strongly fringing intermediate saidinlet and ,said outlet so that the total radial components of'References Cited bythe Examiner UNITED STATES PATENTSV the magneticlines Vof force of said second magnetic 2,946,914 Y 7/ 1960 Colgate etal. 313-231 eld greatly predominate over the axial components 10v2,992,345 7/1961 Hausen V 315-111 X Y ofY the magnetic lines of forceof said second mag- 3,029,635 4/19'62 Petz V 3133+231V X netic field andVcross the current density lines of the axial current low of said plasmajet through said channel, the field strength of said second magnetic Y teld being sufficiently strong with respect to the am- 15 DAVID J.GALVIN, Examiner.

GEORGE N. WESTBY, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,243,954 April 5, 1966 Gordon L. Cann It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below: Column l, after line l2, insert thefollowing paragraph:

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958 Public Law 85-568 (72 Stat.435 42 U. S.C. Z457) Signed and sealed this 2nd day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, Ir.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

1. A PLASMA ACCELERATOR COMPRISING, IN COMBINATION: (A) A CASING HAVINGAN AXISYMMETRIC SUPERSONIC EXPANSION NOZZLE THEREIN EXTENDINGLONGITUDINALLY BETWEEN A SONIC ORIFICE INLET AND A NOZZLE OUTLET, THELONGITUDINAL AXIS OF SAID NOZZLE DEFINING A REFERENCE AXIS FOR THEACCELERATOR; (B) MEANS FOR AT LEAST PARTIALLY IONIZING A GAS STREAM ANDINJECTING THE RESULTING GAS PLASMA AXIALLY INTO THE SONIC ORIFICE INLETOF SAID NOZZLE; (C) MEANS FOR ESTABLISHING AN AXISYMMETRIC ELECTRICDISCHARGE EXTENDING AXIALLY THROUGH SAID NOZZLE TO THEREBY ESTABLISH APLASMA JET DEFINING AN AXIAL CURRENT FLOW THROUGH SAID NOZZLE; AND (D)MEANS FOR ESTABLISHING A MAGNETIC FIELD WITHIN AT LEAST A PREDETERMINEDLONGITUDINAL PORTION OF SAID NOZZLE, SAID MAGNETIC FIELD BINGAXISYMMETIC WITH RESPECT TO SAID REFERENCE AXIS AND STRONGLY FRINGINGINTERMEDIATE SAID SONIC ORIFICE INLET AND SAID NOZZLE OUTLET SO THAT THETOTAL RADIAL COMPONENTS OF THE MAGNETIC LINES OF FORCE OF SAID MAGNETICFIELD GREATLY PREDOMINATE OVER THE AXIAL COMPONENTS OF THE MAGNETICLINES OF FORCE OF SAID MAGNETIC FIELD AND CROSS THE CURRENT DENSITYLINES OF THE AXIAL CURRENT FLOW OF SAID PLASMA JET THROUGH SAIDACCELERATOR CHANNEL.